There exists more that 60,000 aircraft in the world that have auxiliary power units (hereinafter, APU's) to provide auxiliary electric power to the instruments and other equipment on the aircraft and in many cases high volumes of low pressure compressed air to start jet engines. In most cases the aircraft cannot take off if the APU is not functioning properly. These units typically are very reliable but like all equipment they are subject to failure.
To determine the cause of a failure of an APU, maintenance personnel typically use standard electronic test equipment such as voltmeters, ammeters, and oscilloscopes. A typical APU is the Turbomach T40LC-2 APU (hereinafter, T40LC-2 APU) provided for the United States Air Force KC135R aircraft and other aircraft. There are about 586 of these units currently installed in operating aircraft. The value of these aircraft is as much as $10 million each and the total value of all of these aircraft is over $8 billion. If APU's in these very valuable aircraft are not working the aircraft does not take off.